Although progress has been made in targeting various genomic loci, there still remain many types of genomic loci that cannot be targeted efficiently or genomic modifications that cannot be achieved properly or efficiently with conventional targeting strategies. For example, difficulties arise when attempting to create large targeted genomic deletions or other large targeted genetic modifications, particularly in eukaryotic cells and organisms.
In particular, it is difficult to efficiently produce cells or animals that are homozygous or compound heterozygous (e.g., hemizygous) for a large targeted genomic deletion or other genomic modification when using conventional targeting strategies. For example, although F0 generation mice heterozygous for a large targeted genomic deletion are obtainable via conventional targeting strategies, subsequent breeding of these heterozygous mice is required to produce F2 generation mice that are homozygous for the deletion. These additional breeding steps are costly and time-consuming.